[AndrewJ]

I don't think it'll help.

A flywheel is just going to help engine momentum, if you're using P&G then you don't really want more engine momentum.

Plus, from a more theoretical standpoint, a flywheel is an energy storage device. So you're storing your gasolines energy in kinetic form.
You're also losing energy through mechanical friction (of the flywheel) and you're losing energy every time you have to spin the heavier flywheel up to speed.

Same thing as lighter wheels really. I know there's some debate about lighter wheels, but I don't think anyone's in a big rush to go put heavier wheels on their car for the sake of FE.

I can appreciate the "whatever's good for racing is bad for FE" idea, but I don't think it applies in this case.